This invention relates in general to electromagnetic transducers and in particular to a new and useful electromagnetic transducer for converting mechanically recorded acoustic events into alternating voltages particularly for tracing stereo signals recorded on plate shaped supports. The transducer of the invention includes a substantially cylindrical radially magnetized permanent magnet arranged around four pole rods which are disposed with their ends adjacent a soft iron piece connected to a pivotally mounted stylus.
Prior art transducer systems are provided with an easily magnetizable (soft) iron shell and a usually disc-shaped permanent magnet disposed at one end thereof. At the other end of the soft iron shell, an air gap is provided between the pole rods and a soft iron element which is usually designed as a small tube and driven by the pickup stylus. To minimize signal distortion, the soft iron tube is magnetized to, or close to, the saturation point. The windings provided on the pole rods serve to convert the magnetic flux, which varies in accordance with the recorded acoustic event, to proportional voltages. The advantage of such a transducer system is the small mass to be moved, including the stylus, its support, and the soft iron tube. In the prior art, the large number of individual parts needed and the laborious assembly resulting therefrom are disadvantages which make the manufacture of such systems relatively expensive. Another disadvantage is the necessity of magnetizing the soft iron tube carried on the stylus support to the range of saturation, to minimize distortion. This calls for a permanent magnet having a certain minimum mass and thus dimensions which make it rather difficult to incorporate the magnet in conventional constructions. A further disadvantage is that, in the area of the soft iron element which is driven by the stylus through the support thereof, only a relatively small stray flux occurs between the pole rods and the soft iron tube, with the result that most frequently the soft iron tube cannot be magnetized to the saturation point and some allowance must be made for distortion which depends on the degree of saturation. To obtain the necessary sensitivity in the prior art system, windings with a great number of turns are needed on the pole rods. This results in a high internal inductance of the windings and in a disadvantage that if such pickups are connected to an amplifier, through cables having a higher capacitance, resonance peaks are produced causing inadmissible sound distortions in the audible range.
A transducer is also known in which a soft iron tube is disposed within an annular permanent magnet which is magnetized in the axial direction. This arrangement requires a pole plate for supporting the pole rods and for conducting the magnetic flux from the permanent magnet to the pole rods. Since, in this design, the soft iron element is located in the area of the front ends of the pole rods, magnetic saturation of the soft iron tube can be attained in practice, however, additional flux conducting elements are needed which complicate the design of such prior art system. Since, due to its axial magnetization, the permanent magnet is effective as if it were disposed at one end of the pole rods, no compensation of uni-directional fields can be provided, and the pole rods are subjected to a considerable bias magnetization. Also, the dimensions of the permanent magnet remain limited, which requires windings with a great number of turns on the pole rods. The drawbacks connected thereto have already been mentioned above.